Field of the Invention
The present invention relates generally to circuit boards and communication systems, and in particular to structures and methods for differential signal channel length compensation in communication systems.
Background Art
Data transmission speed requirements have dramatically increased over recent years. Electronic systems typically use printed circuit boards that are interconnected to produce various functionalities of the data transmission process. Each printed circuit board contains two or more layers upon which components are placed. Connectivity between printed circuit boards and external systems proceeds by way of connectors. Connectivity between individual components on a printed circuit board is accomplished by way of conductive paths that are commonly referred to as metal traces, or just traces.
With the increased data transmission speed requirements and the need for reduced circuit board size, the type of traces and the routing of those traces within the circuit board are important design considerations. Smaller circuit board size requires the placement of traces in close proximity with one another. However, the close proximity of traces to one another increases the possibility of noise generation and crosstalk between the traces.
One way of avoiding the above problems is to use differential signals. Differential signals are signals that are represented by a pair of signal traces. The voltage difference between the pair of signal traces represents the desired signal. Thus, if external noise or cross talk from a neighboring trace couples into the pair of signal traces, then the coupling would be similar in both of the signal traces. Consequently, the difference in those signal traces remains relatively unaffected and thus immunity to electromagnetic interference (EMI) is enhanced.
Differential signals require that the individual traces of each differential pair be as identical as possible when routed across the printed circuit board. In particular, it is desirable that the lengths of individual traces in a pair be reasonably identical to each other. Differences in the lengths of individual traces in a differential pair result in undesired timing differences in the individual signals. Thus, one signal in the shorter trace of the pair arrives earlier than the signal in the other trace of the pair. Consequently, although the two signals are nominally equal and opposite, the differential signal is degraded by the timing difference or skew in the trace pair. Length mismatches are difficult to avoid in PCB design due to various factors including constraints of PCB stackup, PCB size, component placement, routing density and routing length.
Although the above considerations require identical trace lengths within a differential pair, such a requirement is difficult to meet. Real estate on modern day printed circuit boards is a premium and the ability to route differential traces so that both traces are identical in length faces numerous obstacles. Thus, a circuit layout design can result in unequal trace lengths based on meeting other circuit objectives. Moreover, traditional methods of length matching, such as serpentine routing, do not work well in a multi-gigaHertz differential signaling environment since these traditional methods impair the differential property of the differential trace pair.